Add npm-cache to Travis build
Add npm-cache to Travis build

This seems to be a more efficient caching strategy for npm which will
hopefully speed up our builds.

djvoa12/frontend,ilios/frontend,jrjohnson/frontend,gboushey/frontend,dartajax/frontend,gabycampagna/frontend,gabycampagna/frontend,gboushey/frontend,ilios/frontend,djvoa12/frontend,jrjohnson/frontend,thecoolestguy/frontend,dartajax/frontend,thecoolestguy/frontend